At present, the broadcasting of digital television programmes by satellite channel (for example according to one of the standards DVB-S, DVB-S2 or DVB-SH) is widely used throughout the world. Numerous devices are installed in millions of user's homes.
The installed devices are predominately reception devices which comprise an external unit including a parabolic reflector which focuses the modulated hyperfrequency signals, onto the source, called feedhorn, of an LNB (Low Noise Block), the LNB transforming the hyperfrequency signals received into electrical signals in intermediate satellite band in order to transmit them, via a coaxial cable, to the STB satellite decoder.
The decoder comprises a demodulation block (DVB-S, DVB-S2 or DVB-SH) which extracts a “useful” modulated signal in the modulated signal transmitted on the coaxial cable and demodulates the extracted “useful” signal. The “useful” demodulated signal may, for example, be used to display video images on a television screen.
Offers for broadcasting digital television programmes via terrestrial or satellite channel are today essentially purely passive, that is to “one-way service”.
It may nevertheless prove to be interesting to be able to offer services requiring a return link; this is the case for example of interactive services (votes, consumption of contents with conditional access by exchange of keys, orders for new services such as video on demand). Moreover, this return link can find particularly interesting applications in the field of machine to machine (or M2M) communications for controlling certain devices (alarms, heating, etc.) and/or for recovering data measured by sensors or meters (gas, electricity, etc.) present within homes.
A known solution to this problem consists in using a return link employing an ADSL type connection supplied by fixed telephone operators (STN or “Switched Telephone Network”) or a GPRS/UMTS type connection supplied by mobile telephone operators. This solution thus requires additional equipment as well as an additional subscription; furthermore, telephone switching is not particularly suited to the transmission of not very large messages such as vote or order messages (relatively high cost, problems of saturation of the network, etc.). Furthermore, such a solution is not entirely satisfactory in so far as the interactive functionalities are not based on the same type of installation and equipment already used for the reception of television by satellite.
Most satellite television offers do not include a return link. Interactivity by satellite may be offered as a system completely separate from the broadcasting system, but with quite high costs and not very compatible with a television offer. It is possible nevertheless to cite an example of two-way system for broadcasting television by satellite described in the patent application WO2011076791 filed by the applicant. In this system, the terminals are low cost and the interactive sub-system is integrated with the broadcasting sub-system. This system uses a forward link in wide Ku or Ka band and a return link for the terrestrial terminals in a band different to the Ku band, for example in narrow S band or in C band.
A return link in the Ku band could also be conceived, but in a portion of the Ku band that is not used by the forward link, so that the forward link is not scrambled by the signal in the return link.
It may nevertheless prove to be particularly interesting to use the same portion of band in forward link and in return link; such a configuration enables in fact better spectral efficiency and thus assures a reduction in terms of costs in so far as a same transponder may be used for the forward link and for the return link, without having need of a dedicated transponder for the return link.
Such a configuration is particularly described in the U.S. Pat. No. 6,011,952 in which a main terrestrial station transmits in forward link in a wide band whereas terminals transmit at lower power in return link in sub-bands separated from each other and present in the wide band used for the forward link. According to this configuration, the transmitting terrestrial station is also in charge of the elimination of the main signal transmitted by itself and the recovery of the signals transmitted by the terminals. The fact of using the same terrestrial station for the transmission of the main signal and the recovery of the signals in return link leads to a lack of flexibility of the overall architecture.
The elimination technique proposed in the document U.S. Pat. No. 6,011,952 assumes that the amplification of the signals at the level of the relay device (which may be the transponder of a satellite) is carried out with an amplification chain of the transponder operating uniquely in linear regime. Such an operation with exclusively linear effects thus implies that the amplification chain (including for example travelling wave tube amplifiers or TWTA) of the transponder cannot operate close to saturation or at saturation (situations in which non-linear effects appear). The use of an operation at saturation can however prove to be particularly interesting (or even indispensable) when broadcast type signals such as TV signals are transmitted. In fact, for an application such as television broadcasting, it is very important to minimise the size of the receiving antenna and to maximise the resistance to rain (given by the margin in the link budget). To obtain these effects, the satellite transponder must be used very close to saturation. For memory, the amplifiers used in satellite transponders have a non-linear behaviour when they are used with high power levels (power saturation). These non-linear effects can produce a distortion of the output signal, harmonic frequencies or intermodulation signals in the case of multicarriers. In the linear domain of a travelling wave, the output power at the end of a tube is proportional to the input power. Saturation arises when the output power no longer increases linearly with the input power. In other words, the input power over output power characteristic, also designated by amplitude/amplitude (AM/AM) modulation is not linear when the output of the amplifier is maximum.
Apart from the effects of gain compression, power amplifiers also have non-linear amplitude/phase (AM/PM) modulation effects.
The saturation of the output power goes hand in hand with a change of phase of the wave. When one remains in the linear domain, the variation of the dephasing with the input power remains low, but when saturation is approached, thus the maximum power that the amplifier can supply, the two effects of AM/PM conversion (“AM/PM conversion”—kp) and AM/PM transfer (“AM/PM transfer”—kt) are going to manifest themselves.
The presence of these strong effects due to the saturation of the satellite transponder prevents the use of the elimination technique described in the document U.S. Pat. No. 6,011,952 in the context of a satellite television broadcasting system.